Small cell beam-forming antennas

ABSTRACT

A small cell cellular base station includes an eight port radio and an eight-port base station antenna that has four linear arrays of dual-polarized radiating elements. Each of the linear arrays has a different azimuth boresight pointing direction and each dual-polarized radiating element includes first and second radiators that have respective directional radiation patterns. The radio is configured to determine and apply a first set of amplitude and phase weights to RF signals that are received through the eight ports of the antenna, and to apply a second set of amplitude and phase weights to RF signals that are output by the radio to the eight ports of the antenna.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.62/519,370, filed Jun. 14, 2017 and entitled “SMALL CELL BEAM-FORMINGANTENNAS,” the entire contents of which are incorporated by referenceherein for all purposes.

FIELD

The present invention relates to cellular communications systems and,more particularly, to base station antennas for small cell basestations.

BACKGROUND

Cellular communications systems are well known in the art. In a typicalcellular communications system, a geographic area is divided into aseries of regions that are referred to as “cells,” and each cell isserved by a “macrocell” base station. Each cell may, for example, havean area on the order of 1-50 km², with the cell size depending upon,among other things, the terrain and population density. The base stationmay include baseband equipment, radios and antennas that are configuredto provide two-way radio frequency (“RF”) communications with fixed andmobile subscribers (“users”) that are positioned throughout the cell.The base station antennas are often mounted on a tower or other raisedstructure, with the radiation beam (“antenna beam”) that is generated byeach antenna directed outwardly to serve the entire cell or a portion(“sector”) thereof. Typically, a base station antenna includes one ormore phase-controlled arrays of radiating elements, with the radiatingelements arranged in one or more vertical columns when the antenna ismounted for use. Herein, “vertical” refers to a direction that isgenerally perpendicular relative to the plane defined by the horizon.

In order to increase capacity, cellular operators have been deployingso-called “small cell” base stations. A small cell base station refersto a lower power base station that may operate in the licensed and/orunlicensed spectrum that serves a much smaller area than a typicalmacrocell base station. Herein, the term “small cell” is used broadly torefer to base stations that serve smaller areas than conventionalmacrocell base stations, and thus the term “small cell” encompassessmall cell, microcell, picocell and other base stations that serve smallgeographic regions. Small cell base stations may be used, for example,to provide cellular coverage to high traffic areas within a macrocell,which allows the macrocell base station to offload much or all of thetraffic in the vicinity of the small cell to the small cell basestation. Small cell base stations may be particularly effective in LongTerm Evolution (“LTE”) cellular networks in efficiently using theavailable frequency spectrum to maximize network capacity at areasonable cost.

FIG. 1 is a schematic diagram of a conventional small cell base station10. The base station 10 includes an antenna 20 that may be mounted on araised structure 30. In the depicted embodiment, the structure 30 is asmall antenna tower, but it will be appreciated that a wide variety ofmounting locations may be used including, for example, utility poles,buildings, water towers and the like. Typically, the antenna 20 of asmall cell base station is designed to have an omnidirectional antennapattern in the azimuth plane, meaning that the antenna beam generated bythe antenna 20 may extend through a full 360 degree circle in theazimuth plane, and may have a suitable beamwidth (e.g., 10-30 degrees)in the elevation plane. The antenna beam may be slightly down-tilted inthe elevation plane (which may be a physical or electronic downtilt) toreduce spill-over of the antenna beam of the small cell base stationantenna into regions that are outside the small cell and also forreducing interference between the small cell base station and theoverlaid macrocell base station.

The small cell base station 10 further includes base station equipmentsuch as one or more baseband units 40 and radios 42. While the radio 42is shown as being co-located with the baseband equipment 40 at thebottom of the antenna tower 30, it will be appreciated that in othercases the radio 42 may be a remote radio head that is mounted on theantenna tower 30 adjacent the antenna 20. As is known to those of skillin the art, the baseband unit 40 may receive data from another sourcesuch as, for example, a backhaul network (not shown) and may processthis data and provide a data stream to the radio 42. The radio 42 maygenerate RF signals that include the data encoded therein and mayamplify and deliver these RF signals to the antenna 20 for transmissionvia a cabling connection 44. It will also be appreciated that the smallcell base station 10 of FIG. 1 may typically include various otherequipment (not shown) such as, for example, a power supply, back-upbatteries, a power bus, controllers and the like.

SUMMARY

Pursuant to embodiments of the present invention, small cell basestations are provided that include a base station antenna having firstthrough eighth ports and first through fourth linear arrays ofdual-polarized radiating elements, where at least two of the firstthrough fourth linear arrays have different azimuth boresight pointingdirections when the base station antenna is mounted for use, and whereineach dual-polarized radiating element includes first and secondradiators that have respective directional radiation patterns. The smallcell base station further includes a radio having first through eighthradio ports that are connected to the respective first through eighthports of the base station antenna. The radio is configured to determineand apply a first set of amplitude and phase weights to RF signals thatare received from the respective first through eighth ports of the basestation antenna, and to apply a second set of amplitude and phaseweights to RF signals that are output by the radio to the first througheighth ports of the base station antenna.

In some embodiments, the amplitude and phase weights of the first andsecond sets of amplitude and phase weights may be determined on a timeslot-by-time slot basis.

In some embodiments, the amplitude and phase weights of the second setof amplitude and phase weights may be the complex conjugates of therespective amplitude and phase weights of the first set of amplitude andphase weights.

In some embodiments, the azimuth boresight pointing direction of thefirst linear array may be offset from the azimuth boresight pointingdirection of the second through fourth linear arrays by about 90degrees, about 180 degrees and about 270 degrees, respectively.

In some embodiments, the base station may further include a feed networkthat connects all of the first radiators of each of the first throughfourth linear arrays to the first through fourth radio ports,respectively, and that connects all of the second radiators of each ofthe first through fourth linear arrays to the fifth through eighth radioports, respectively.

In some embodiments, the base station antenna may be configured totransmit as a multi-input-multi-output antenna using the first radiatorsto transmit a first data stream and using the second radiators tosimultaneously transmit a second data stream that is different than thefirst data stream. In such embodiments, the first and second datastreams may be part of a composite data stream that is transmitted to auser.

In some embodiments, the base station antenna may further include firstthrough fourth backplanes that together define a tubular reflectorassembly. The tubular reflector assembly may have, for example, agenerally rectangular cross-section in the azimuth plane.

In some embodiments, the base station antenna may further include aninth port and a calibration element that is connected to the ninthport, where the ninth port of the base station antenna is connected to aninth radio port.

In some embodiments, the first linear array may point in a firstdirection and the third linear array may point in a third direction thatis substantially opposite the first direction. In such embodiments, thesecond linear array may point in a second direction and the fourthlinear array may point in a fourth direction that is substantiallyopposite the second direction. The first direction may be angularlyoffset from the second direction by about 90 degrees in someembodiments. In other embodiments, the first direction may be the sameas the second direction.

Pursuant to further embodiments of the present invention, base stationantennas are provided that include first through eighth ports and aplurality of radiating elements that are arranged as first throughfourth linear arrays of radiating elements, each radiating elementhaving a half-power azimuth beamwidth of less than 120 degrees andcomprising a first radiator that radiates at a first polarization and asecond radiator that radiates at a second polarization that isorthogonal to the first polarization, where the radiating elements ofthe first linear array are mounted to have a first azimuth boresightpointing direction, the radiating elements of the second linear arrayare mounted to have a second azimuth boresight pointing direction, theradiating elements of the third linear array are mounted to have a thirdazimuth boresight pointing direction, and the radiating elements of thefourth linear array are mounted to have a fourth azimuth boresightpointing direction, where at least two of the first through fourthazimuth boresight pointing directions differ from each other. These basestation antennas further include a feed network that connects each ofthe first polarization radiators of the first linear array to the firstport, connects each of the second polarization radiators of the firstlinear array to the second port, connects each of the first polarizationradiators of the second linear array to the third port, connects each ofthe second polarization radiators of the second linear array to thefourth port, connects each of the first polarization radiators of thethird linear array to the fifth port, connects each of the secondpolarization radiators of the third linear array to the sixth port,connects each of the first polarization radiators of the fourth lineararray to the seventh port, and connects each of the second polarizationradiators of the fourth linear array to the eighth port.

In some embodiments, the first through fourth azimuth boresight pointingdirections may differ from each other by at least 50 degrees. In anexample embodiment, the first through fourth azimuth boresight pointingdirections may be approximately 0 degrees, 90 degrees, 180 degrees and270 degrees, respectively. The base station antenna may be provided incombination with an eight port beam-forming radio, where the firstthrough eighth ports of the base station antenna are coupled torespective first through eighth radio ports of the beam-forming radio.The base station antenna may be configured to operate in both abroadcast mode in which the base station antenna generates anomnidirectional antenna beam and in a service beam mode in which thebase station antenna generates at least one directional antenna beam. Inthe service beam mode the first through fourth linear arrays may beconfigurable to simultaneously generate at least four independentantenna beams. In the broadcast mode the beam-forming radio may beconfigured to subdivide an RF signal into eight identical sub-componentsand to output the sub-components through the respective first througheighth radio ports.

In some embodiments, the base station antenna may further include aninth port and a calibration element that is connected to the ninthport, where the ninth port of the base station antenna is connected to aninth radio port of the beam-forming radio.

Pursuant to still further embodiments of the present invention, methodsof operating a base station antenna are provided. The base stationantenna includes a plurality of linear arrays of dual-polarizedradiating elements, at least two of the linear arrays having differentazimuth boresight pointing directions. Pursuant to these methods, an RFsignal is received from a user through the plurality of linear arraysduring a first time slot of a time division multiplex system. Thereceived RF signal is passed to a beam-forming radio. A first set ofamplitude and phase weights are determined, the first set of amplitudeand phase weights including a respective amplitude weight and arespective phase weight for each linear array at each of a firstpolarization and a second polarization. The first set of amplitude andphase weights are applied to the received RF signals. A second set ofamplitude and phase weights are determined, the second set of amplitudeand phase weights including a respective amplitude weight and arespective phase weight for each linear array at each of the firstpolarization and the second polarization. The second set of amplitudeand phase weights are then applied to one or more RF signals that aretransmitted to the user through the plurality of linear arrays.

In some embodiments, the plurality of linear arrays may comprise fourlinear arrays, and at least two of the four linear arrays may point inopposite directions.

In some embodiments, the plurality of linear arrays comprises fourlinear arrays, and the azimuth pointing directions of each linear arraymay be offset from the azimuth pointing directions of each of the threeother linear arrays by about 90 degrees, 180 degrees and 270 degrees,respectively.

In some embodiments, the antenna may include a total of four lineararrays and first through eighth ports, where each radiating elementincludes a first radiator that radiates at a first polarization and asecond radiator that radiates at a second polarization that isorthogonal to the first polarization, and the antenna further comprisesa feed network that connects each of the first polarization radiators ofthe first linear array to the first port, connects each of the secondpolarization radiators of the first linear array to the second port,connects each of the first polarization radiators of the second lineararray to the third port, connects each of the second polarizationradiators of the second linear array to the fourth port, connects eachof the first polarization radiators of the third linear array to thefifth port, connects each of the second polarization radiators of thethird linear array to the sixth port, connects each of the firstpolarization radiators of the fourth linear array to the seventh port,and connects each of the second polarization radiators of the fourthlinear array to the eighth port.

In some embodiments, the base station antenna may further include aninth port and a calibration element that is connected to the ninthport.

In some embodiments, the one or more RF signals that are transmitted tothe user through the plurality of linear arrays may comprise a first RFsignal that is transmitted by the first radiators of the first throughfourth linear arrays and a second RF signal that is transmitted by thesecond radiators of the first through fourth linear arrays.

In some embodiments, the one or more RF signals that are transmitted tothe user through the plurality of linear arrays may comprise a first RFsignal that is transmitted by the first radiators of the first andsecond linear arrays, a second RF signal that is transmitted by thefirst radiators of the third and fourth linear arrays, a third RF signalthat is transmitted by the second radiators of the first and secondlinear arrays, and a fourth RF signal that is transmitted by the secondradiators of the third and fourth linear arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a highly simplified schematic diagram illustrating aconventional small cell base station.

FIG. 2 is a collage including a schematic front view, top view andperspective view of the linear arrays of a prior art base stationantenna.

FIG. 3 is a collage including a schematic front view, top view andperspective view of a conventional beamforming small cell base stationantenna with the radome thereof partially shown in the perspective view.

FIG. 4 is a perspective view of a conventional panel style beam-formingantenna with the radome removed.

FIG. 5A is a schematic diagram illustrating a beam-forming small cellbase station antenna according to embodiments of the present invention.

FIGS. 5B and 5C are graphs illustrating azimuth and elevationcross-sections of the antenna beam of the small cell antenna of FIG. 5A.

FIG. 6A is a schematic diagram illustrating a small cell base stationantenna according to embodiments of the present invention.

FIGS. 6B-6J are graphs illustrating azimuth or elevation cross-sectionsof antenna beams that may be generated by exciting various columns orcombinations of columns of the small cell antenna of FIG. 6A.

FIGS. 7A and 7B are a side view and a top view, respectively, of two ofthe radiating elements included in the base station antenna of FIG. 6A.

FIG. 8 is a block diagram illustrating a feed network that may beincluded in the base station antenna of FIG. 6A.

FIG. 9 is a block diagram of a base station antenna according toembodiments of the present invention that has a calibration port and adistributed calibration element.

FIG. 10 is a flow chart diagram illustrating a method of operating abase station antenna according to embodiments of the present invention.

FIG. 11 is a schematic perspective view of a small cell base stationantenna according to further embodiments of the present invention.

DETAILED DESCRIPTION

Pursuant to embodiments of the present invention, small cellbeam-forming base station antennas are provided that are suitable foruse in LTE time division duplex (“TDD”) and frequency division duplex(“FDD”) systems. The small cell beam-forming antennas according toembodiments of the present invention may have a very small form factorand may be mounted on light posts, electric power poles, telephonespoles and the like. These small cell beam-forming antennas may providefull 360° coverage in the azimuth plane using multiple linear arrays or“columns” of radiating elements that have directional radiationpatterns. The small cell antennas according to embodiments of thepresent invention may form directional antenna beams having relativelyhigh gain on a time slot-by-time slot basis and may be used inmulti-input-multi-output (“MIMO”) and/or beam-forming modes ofoperation.

In some embodiments, the beam-forming antennas according to embodimentsof the present invention may include four linear arrays of radiatingelements that are mounted on the four main faces of a rectangulartubular reflector assembly. The azimuth boresight pointing direction ofeach linear array may be offset by approximately 90° from the azimuthboresight pointing directions of two adjacent linear arrays. Theradiating elements in each linear array may comprise dual-polarizedradiating elements such as, for example, slant −45°/+45° cross-dipoleradiating elements. The radiating elements may have directional patternsin the azimuth plane having, for example, azimuth half power beamwidthsof between 50-120°. Each of the four linear arrays may connect to twoports (one for each polarization) on the antenna, and the eight ports onthe antenna may connect to corresponding radio ports on an eight-portbeam-forming radio. Each linear array may form a pair of directionalantenna beams, one for each orthogonal polarization. Each antenna beammay, for example, provide coverage for over ninety degrees in theazimuth plane.

Digital beamforming may be performed so that RF signals are transmittedand received through two or more of the linear arrays, with appropriateamplitude and phase weighting provided for each linear array, in orderto generate a relatively narrow antenna beam in the azimuth plane thathas a high gain. For example, the same RF signal may be provided to twoadjacent linear arrays, with the sub-component of the RF signal that isfed to each linear array amplitude and phase weighted so that togetherthe two linear arrays form a single antenna beam that is pointed at auser that is located between the azimuth boresight pointing directionsof the two linear arrays. The remaining two linear arrays may also beexcited (potentially at much lower power levels) in order to generateone or more nulls within the antenna beam in the location(s) of sourcesof potentially interfering signals, or to provide a second antenna beamthat illuminates the user by reflecting off various structures.

The antennas according to embodiments of the present invention may beused in LTE-TDD systems as small cell base station antennas. Whileeight-port beam-forming antennas are currently used in LTE-TDD systems,these antennas are “panel” antennas that have four columns ofdual-polarized radiating elements mounted on a common, planar backplaneso that all four columns face in the same direction (i.e., have the sameazimuth boresight pointing direction). These panel antennas aretypically used as sector antennas in macrocell base station antennas andmay not be well-suited for many small cell applications—particularly indense urban environments—where omnidirectional coverage is oftendesired. In contrast, the beam-forming antennas according to embodimentsof the present invention may have a plurality of directional (and hencehigher gain) linear arrays that together provide omnidirectionalcoverage.

The small cell antennas according to certain embodiments of the presentinvention may be used to as MIMO antennas and/or as beam-formingantennas. For example, the above-described four column antenna havinglinear arrays of dual-polarized radiating elements that are offset by90° in the azimuth plane may be used as a MIMO antenna for users thatare close to the small cell base station and as a beam-forming antennafor serving users near the outer edge of the small cell. In oneconfiguration, the antenna may implement 2×MIMO using the two orthogonalpolarizations and use all four columns at each polarization forbeam-forming to create high gain directional beams in the azimuth plane.In another configuration, the antenna may implement 4×MIMO using thefirst and second columns (at each of two orthogonal polarizations) totransmit first and second MIMO data streams and using the third andfourth columns (at each of two orthogonal polarizations) to transmitthird and fourth MIMO data streams. The sub-components of an RF signalthat are fed to the first and second columns may be amplitude-weightedand phase-weighted to narrow the two antenna beams formed by the firstand second columns, and the third and fourth columns may likewise beamplitude-weighted and phase-weighted to narrow the antenna beam formedby the third and fourth columns.

The small cell antennas according to embodiments of the presentinvention may be used in LTE Transmission Mode 8 (“LTE TM8 mode”), whichis a dual layer beam-forming mode. A conventional beam-forming radio maybe used with such an antenna to demodulate and process the RF signalsreceived from a user during a particular TDD time slot to determineamplitude and phase weights for the signals received at the eight portsof the antenna that will result in a combined received signal thatoptimizes a parameter such as the received signal-to-noise ratio, biterror rate or the like. The complex conjugates of the amplitude andphase weights may then be used to generate amplitude and phase weightsfor RF signals that are transmitted from the base station to thatparticular user. A similar process may be used in FDD systems exceptthat the downlink amplitude and phase weights may be further modified tocorrect for differences in the channel model that result from thedifference between the frequencies used for the uplink and downlinktransmissions.

Several conventional beam-forming antennas as well as exampleembodiments of the invention will now be discussed in more detail withreference to the attached drawings.

Beam-forming antennas refer to antennas that have multiple columns ofone or more radiating elements that are fed by different ports of aradio. A radio may modulate an RF signal and then send it totransceivers for each output port of the radio (“radio port”). Theamplitude and phase of the RF signal for each radio port may be set bythe radio so that the columns of radiating elements work together toform a more focused, higher gain antenna beam that has a narrowedbeamwidth in the azimuth plane (or two such antenna beams if the antennahas dual-polarized radiating elements, namely an antenna beam for eachpolarization). In other cases the antenna may be arranged so that thereare multiple input ports for sub-arrays in the elevation direction aswell as azimuth direction so that the antenna beam may be narrowed inboth the azimuth and elevation planes. The antenna beams may be changedon a time slot-by-time slot basis in a TDD transmission scheme in orderto increase the antenna gain in the direction of selected users duringeach time slot. The column spacing (i.e., the horizontal distancebetween adjacent vertically-oriented linear arrays of radiatingelements) of a beam-forming antenna is typically relatively small. Sincebeam-forming antennas have the ability to narrow the azimuth (andperhaps elevation) beamwidth and to scan the antenna beam in thedirection of a user, they may exhibit higher antenna gains and supportincreased capacity. Beam-forming antennas typically include acalibration network so that the amplitude and phase relationshipsbetween adjacent columns may be tightly controlled. The calibrationnetwork allows the radio to compensate for differences in amplitude andphase between the jumper cables that connect the multiple radio ports tothe corresponding antenna ports.

FIG. 2 is a collage that includes a schematic front view, a top view anda perspective view of a prior art multi-column small cell antenna 50. Asshown in FIG. 2, the antenna 50 includes eight linear arrays 52 ofvertically-disposed omnidirectional dipoles (the individual dipoles arenot visible in FIG. 2). Each linear array 52 had its own radome 54 toprovide environmental protection, and the eight radomes 54 are arrangedto define a cylinder. The antenna 50 is a dual-band antenna, with fourof the linear arrays 52 configured to operate at 1.9 GHz and the otherfour linear arrays 52 configured to operate at 2.6 GHz. The diameter ofthe antenna 50 is about six feet, which allowed each linear array 52 tobe highly decorrelated with respect to the other linear arrays 52operating in the same frequency band. However, given the large spacingbetween the linear arrays 52, the antenna 50 was not operable as abeam-forming antenna. The antenna 50 used spatial diversity to improveperformance.

FIG. 3 is a collage of a top view, a side view and a shadow perspectiveview of another conventional small cell beam-forming antenna 60 that wasdesigned to operate in 3G TD-SCMA systems. As shown in FIG. 3, thebeam-forming antenna 60 has eight columns (or linear arrays) 62 ofvertically polarized radiating elements 64 that are arranged in anoctagon around the circumference of a support structure 66. The lineararrays 62 are spaced sufficiently close together so that the antenna 60can use beam-forming techniques to feed multiple columns together toform narrowed antenna beams. A circular radome 68 is mounted over thesupport structure 66 and the linear arrays 62 to provide environmentalprotection. The individual radiating elements 64 have omnidirectionalpatterns, and hence the antenna 60 could not form high directivityantenna beams. Each linear array 62 of radiating elements 64 is drivenat full power.

Later beam-forming antennas were implemented as panel antennas. Theseantennas typically included multiple columns of radiating elementsmounted to extend forwardly from a planar back plane. FIG. 4 is aperspective view of one such beam-forming panel antenna 70. As shown inFIG. 4, the beam-forming antenna 70 has four columns 72 ofdual-polarized radiating elements 74 that are mounted on a planarbackplane 76. Each column 72 of radiating elements 74 has the sameazimuth boresight pointing angle. The antenna 70 includes a total ofeight ports 78, namely two ports for each column (a port for eachpolarization), along with a ninth port 78 for calibration. A radome (notshown) is mounted over the radiating elements 74 to provideenvironmental protection. Panel beam-forming antennas such as theantenna 70 of FIG. 4 may provide good performance. These panel antennasmay be used as sector antennas on a macrocell base station or may bemounted on the sides of buildings or other structures. There are manysmall cell applications where full 360 degree coverage in the azimuthplane is desired. Panel beam-forming antennas tend to be unsuitable forsuch applications as they do not provide omnidirectional coverage in theazimuth plane.

One possible technique for fabricating a beam-forming antenna thatprovides omnidirectional coverage would be to mount two side-by-sidevertically-oriented linear arrays of radiating elements on each side ofa tube having a triangular horizontal cross-section. This approach, inessence, provides three separate beam-forming antennas that each providecoverage to a 120° sector in the azimuth plane to provide full 360°coverage. An example of such a beam-forming antenna 100 is illustratedin FIG. 5A.

As shown in FIG. 5A, the beam-forming antenna 100 has a tubulartriangular reflector assembly 110 that includes three backplanes 112-1,112-2, 112-3. Six vertically-oriented linear arrays 120-1 through 120-6of dual-polarized radiating elements 122 are mounted on the reflectorassembly 110, with two linear arrays 120 provided per backplane 112.Each backplane 112 may comprise, for example, a reflector that serves asa ground plane for the radiating elements 122 of the linear arrays 120mounted thereon. A single monolithic structure (e.g., a sheet of metalthat is bent to form a tube having a triangular horizontalcross-section) may be used to form all three backplanes 112. Herein,when multiple like or similar elements are provided, they may belabelled in the drawings using a two part reference numeral. Suchelements may be referred to herein individually by their full referencenumeral (e.g., backplane 112-2) and may be referred to collectively bythe first part of their reference numeral (e.g., the backplanes 112).

Two vertically-oriented linear arrays 120 are mounted on each backplane112. In the depicted embodiment, each linear array 120 includes a totalof six dual-polarized radiating elements 122. The dual-polarizedradiating elements 122 may extend forwardly from the respectivebackplanes 112. The linear arrays 120 on each backplane 112 may bespaced close together. For example, the linear arrays 120 on eachbackplane 112 may be spaced apart by about a half wavelength of a centerfrequency of the operating frequency range of the linear arrays 120.

The antenna 100 may have two ports for each linear array 120 (one foreach polarization, for a total of twelve ports. The two linear arrays120 on each backplane 112 may perform independent beam-forming in someembodiments, while beam-forming may be performed using all six lineararrays 120 in other embodiments.

In small cell applications in dense urban environments, RF signalstransmitted and received by the antenna 100 may be subject to a highdegree of scattering. Such scattering tends to enhance the degree ofdecorrelation between adjacent ones of the linear arrays 120, allowinglinear arrays that are spaced closely together (as is desired whenbeam-forming is performed) to also operate in a MIMO mode (which needsdecorrelated antennas, and hence greater spatial separation). Forexample, when used in a dense urban environment, the antenna 100 mayoperate as a 2×MIMO antenna by using each polarization to transmit adifferent data stream. The two linear arrays 120 on each backplane 112may operate together to perform beam-forming. In some cases,beam-forming could be performed using linear arrays 120 from multiplebackplanes, and/or 4×MIMO transmissions could be supported bytransmitting data streams using multiple sets of linear arrays 120(e.g., three linear arrays 120 could be used to generate a first antennabeam at a first polarization and a second antenna beam at a secondpolarization, while the remaining three linear arrays 120 could be usedto generate a third antenna beam at the first polarization and a fourthantenna beam at the second polarization to support 4×MIMO).

FIGS. 5B and 5C are graphs illustrating azimuth and elevationcross-sections of the antenna beam generated by the small cell antennaof FIG. 5A when all six linear arrays 120 are simultaneously excitedin-phase with the same magnitude RF signal. As shown in these graphs,the antenna beam has a quasi-omnidirectional shape in the azimuth planeand has a relatively narrow elevation beamwidth.

It is anticipated that the base station antenna 100 may exhibitrelatively good performance. The base station antenna 100, however, maybe larger and/or more expensive than desirable for some applications. Inparticular, the base station antenna 100 requires a total of six lineararrays 120 with a total of thirty-six radiating elements 122. Such anantenna may be relatively expensive. Additionally, the radome for theantenna 100 will have a diameter on the order of 12 inches. A smallerdiameter radome would be desirable for many small cell applications, asthe utility poles on which small cell base station antennas are commonlymounted typically have a diameter of less than a foot. Moreover, as canbe seen in FIG. 5A, there are three deep nulls in the azimuth plane thatare located at the corners of the triangular reflector assembly 110.Each of these nulls is about 15 dBi below the peak gain value. Whilebeam-forming techniques may reduce the depth of these nulls, it stillmay be difficult to provide coverage for users located in the directionsof these nulls, particularly for users that are at larger distances fromthe antenna 100.

The base station antenna 100 may be connected to a twelve-portbeam-forming radio and may operate as a beam-forming antenna and/or as aMIMO antenna. The discussion below of a beam-forming base stationantenna 200 explains how antennas according to embodiments of thepresent invention may be used in conjunction with an eight-portbeam-forming radio to provide an antenna that can operate inbeam-forming and/or in MIMO modes of operation. The base station antenna100 would operate in an essentially identical manner, with the onlydifference being that base station antenna 100 is a twelve port antennathat works in conjunction with a twelve-port beam-forming radio whilebase station antenna 200 is an eight-port antenna that works inconjunction with an eight-port beam-forming radio. Accordingly, furtherdiscussion of the operation of base station antenna 100 will be omitted.

FIG. 6A is a schematic perspective diagram illustrating a beam-formingbase station antenna 200 according to further embodiments of the presentinvention that is suitable for use as a small cell antenna. As shown inFIG. 6A, the small cell base station antenna 200 includes a rectangulartubular reflector assembly 210. The base station antenna 200 includesfour linear arrays 220-1 through 220-4 of dual-polarized radiatingelements 222. Each face of the reflector assembly 210 may comprise abackplane 212-1 through 212-4. Each backplane 212 may comprise a unitarystructure or may comprise a plurality of structures that are attachedtogether. Each backplane 212 may comprise, for example, a reflector thatserves as a ground plane for the dual-polarized radiating elements 222of the linear arrays 220 mounted thereon.

Each linear array 220 is mounted on a respective one of the backplanes212, and may be oriented generally vertically with respect to thehorizon when the base station antenna 200 is mounted for use so thateach linear array 220 comprises a column of radiating elements 222. Inthe depicted embodiment, each linear array 220 includes a total of sixradiating elements 222. It will be appreciated, however, that othernumbers of radiating elements 222 may be included in the linear arrays220. Each radiating element 222 may be implemented, for example, usingthe radiating element design shown in FIGS. 7A-7B. The base stationantenna 200 further includes a radome 260 that covers and protects theradiating elements 222 and other components of the base station antenna200.

Referring to FIGS. 7A and 7B, in an example embodiment, each lineararray 220 may be implemented as three sub-arrays 221 of radiatingelements 222, where each sub-array 221 includes two radiating elements222 that are mounted on a common feedboard 228. It will be appreciated,however, that sub-arrays 221 may or may not be used in otherembodiments, and that any appropriate radiating elements 222 may beused. It will also be appreciated that different types of radiatingelements 222 may be more suitable for different frequency bands ofoperation.

As is further shown in FIGS. 7A and 7B, each radiating element 222 maycomprise a pair of stalks 224-1, 224-2 and a pair of radiators 226-1,226-2. Each stalk 224 may comprise a microstrip printed circuit board.The two printed circuit boards that form the stalks 224-1, 224-2 may bearranged in an “X” configuration when viewed from above. Each radiator226 may comprise, for example, a dipole. Each radiator 226 may have adirectional pattern in the azimuth plane having, for example, azimuthhalf power beamwidths of between 50°-120°. In the depicted embodiment,the base station antenna 200 is a dual-polarized antenna, and hence eachradiating element 222 includes a pair of dipole radiators 226 arrangedin a so-called “cross-dipole” arrangement, with the first radiator 226being disposed at an angle of −45° from a vertical axis, and the secondradiator 226 being disposed at an angle of +45° from the vertical axis.Each radiator (dipole) 226 may be disposed in a plane that issubstantially perpendicular to a longitudinal axis of its correspondingstalk 224. In some embodiments, both radiators 226-1, 226-2 may beformed on a common printed circuit board. In the depicted embodiment,each sub-array 221 includes a pair of radiating elements 222 that aremounted on a feedboard 228. The feedboard 228 may be configured to splitan RF signal (the split need not be equal) that is provided thereto intotwo sub-components and to feed each sub-component to a respective one ofthe radiating elements 222. The feedboard 228 may include two inputs,namely one for each polarization. Directors 227 may be mounted above theradiators 226 to narrow the beamwidth of the radiating elements 222.

FIG. 8 illustrates an embodiment of a feed network 250 that may be usedto pass RF signals between eight RF connector ports 244 (also referredto herein simply as “ports”) on base station antenna 200 and theradiating elements 222 of the four linear arrays 220. FIG. 8 alsoillustrates the connections between the ports 244 on base stationantenna 200 and the corresponding radio ports 44-1 through 44-8 on aconventional beam-forming radio 42.

As shown in FIG. 8, the base station antenna 200 is an eight portantenna having ports 244-1 through 244-8. While not shown in FIG. 8, thebase station antenna 200 may also have a calibration port that is usedto calibrate a radio so that it can generate amplitude and phase weightsthat will provide desired amplitude and phase shifts between the lineararrays 220. Ports 244-1 through 244-4 are coupled to the −45° dipoles ofthe respective linear arrays 220-1 through 220-4, and ports 244-5through 244-8 are coupled to the +45° dipoles of the respective lineararrays 220-1 through 220-4. Duplexing of the transmit and receivechannels is performed internal to the radio 42 in this particularembodiment.

As shown in FIG. 8, the first port 244-1 is coupled to an input of phaseshifter 280-1. The phase shifter 280-1 may split the RF signals inputthereto three ways (and the power split may be equal or unequal) and mayapply a phase taper across the three sub-components of the split RFsignal to apply an electronic downtilt to the antenna beam that isformed when the sub-components of the RF signal are transmitted (orreceived) through the linear array 220-1. The three outputs of phaseshifter 280-1 are coupled to the −45° polarization transmission lines onthe three feed assemblies 228-1 through 228-3 of linear array 220-1. The−45° polarization transmission lines on each feed assembly 228 include apower splitter (not shown) and the two outputs of each such powersplitter connect to the respective −45° polarization radiators 226 ofthe radiating elements 222 of the respective feed assemblies 228. Thus,an RF signal input at port 244-1 may be split into severalsub-components and then phase shifted, and the phase shiftedsub-components may be split again and fed to the six dipoles 226 oflinear array 220-1 that are arranged to radiate at the −45°polarization. The power splitting performed by the phase shifter 280-1and on the feedboard assemblies 228 may be equal or unequal powersplitting. The number of phase shifter outputs may be different thanthree. In some embodiments, the phase shifters 280 may be integratedonto a monolithic printed circuit board that contains all the radiatingelements, eliminating the need for cables between the phase shifteroutputs and the feedboards 228.

Similarly, the fifth port 244-5 is coupled to an input of phase shifter280-5. The phase shifter 280-5 may split the RF signals input theretothree ways (and the power split may be equal or unequal) and may apply aphase taper across the three sub-components of the split RF signal to,for example, apply an electronic downtilt to the antenna beam that isformed when the sub-components of the RF signal are transmitted (orreceived) through the linear array 220-1. The three outputs of phaseshifter 280-5 are coupled to the +45° polarization transmission lines onthe three feed assemblies 228-1 through 228-3 of linear array 220-1. The+45° polarization transmission lines on each feed assembly 228 include apower splitter (not shown) and the two outputs of each such powersplitter connect to the respective +45° polarization radiators 226 ofthe radiating elements 222 of the respective feed assemblies 228. Thus,an RF signal input at port 244-5 may be split into severalsub-components and then phase shifted, and the phase shiftedsub-components may be split again and fed to the six dipoles 226 oflinear array 220-1 that are arranged to radiate at the +45°polarization.

As shown in FIG. 8, identical feed arrangements may be used to feed theradiating elements 222 of linear arrays 220-2 through 220-4 via phaseshifters 280-2 through 280-4 and 280-6 through 280-8. Therefore, furtherdescription of these portions of the feed network 250 will be omitted.It will also be appreciated that the phase shifters 280 may be omittedin some embodiments, and the RF signals may be split on the feedboardassemblies 228 and coupled to the radiating elements 222. It willlikewise be appreciated that the feedboard assemblies may be omitted insome embodiments and that the radiating elements may be directly fed bycables that are, for example attached to junction boxes. For example, ifdie-cast metal dipoles are used as the radiating elements, the dipolesmay be directly fed by coaxial cables in example embodiments. Thus, itwill be appreciated that any appropriate feed network and radiatingelements may be used, including feed networks that directly feed eachradiating element without the use of any feedboard assemblies.

While FIG. 8 (as well as FIGS. 7A-7B) illustrate an embodiment in whichtwo radiating elements 222 are mounted per feedboard 228, it will beunderstood that any number of radiating elements 222 may be provided perfeedboard 228. For example, in another embodiment, all six radiatingelements 222 may be provided on a single feedboard 228 that couldinclude the phase shifters 280-1 and 280-5 (or, alternatively, the phaseshifters 280 could be omitted) while in other embodiments each radiatingelement 222 could be implemented individually and have a directconnection to outputs of the phase shifters 280-1, 280-5. Manufacturingissues, cost, and the number of radiating elements 222 that can beindividually phase adjusted (assuming that phase shifting for electronicdowntilt control is provided) may be considered in selecting aparticular design.

The base station antenna 200 may further include a number ofconventional components that are not depicted in FIG. 6A or 8. Forexample, a plurality of circuit elements and other structures may bemounted within the reflector assembly 210. These circuit elements andother structures may include, for example, remote electronic tilt (RET)actuators for mechanically adjusting the phase shifters 280, mechanicallinkages and one or more controllers. The antenna 200 may include topand bottom end caps, and the connector ports 244 may be mounted in thebottom end cap. Mounting brackets (not shown) may also be provided formounting the base station antenna 200 to another structure such as anantenna tower or utility pole.

FIGS. 6B-6J are graphs illustrating example beam patterns that may beformed using the small cell antenna 200.

FIGS. 6B and 6C illustrate the azimuth and elevation patterns forantenna 200 when an RF signal is applied to a single port 244 of theantenna, and hence is supplied to the radiators 226 having the firstpolarization of only one of the four linear arrays 220 (FIG. 6Billustrates the case when the RF signal is fed to linear array 220-1,which has an azimuth boresight angle of 0 degrees). As shown in FIG. 6B,the antenna beam has an azimuth half power beamwidth of about 60-65degrees. As shown in FIG. 6C, the elevation pattern has a downtilt ofabout 5 degrees, and an elevation half power beamwidth of about 7degrees.

FIG. 6D is a composite antenna pattern that illustrates the azimuthpattern of FIG. 6B along with the azimuth patterns that are generatedwhen a port that is connected to each of the other three linear arrays220 are excited by themselves. As can be seen, each linear array 220generates a similar azimuth beam pattern, except that the azimuthboresight for each beam pattern is offset by 90 degrees from the beampatterns formed by the two adjacent linear arrays 220. While theelevation patterns corresponding to the azimuth patterns with boresightpointing directions of 90°, 180° and 270° in FIG. 6D are not shown inthe drawings, they are substantially identical to the elevation patternof FIG. 6C.

In many cases, it may be advantageous to use two of the linear arrays220 to form a narrowed antenna beam in the azimuth plane. FIGS. 6E and6F illustrate the azimuth and elevation patterns for antenna 200 when anRF signal is applied to the ports of the antenna that connect to thelinear arrays that have azimuth boresight pointing directions of 0° and90°. In the case of FIGS. 6E and 6F, the signals that are supplied tothe two ports are not amplitude or phase weighted, and hence theresultant azimuth pattern provides hemispherical coverage in the azimuthplane. It will be appreciated, however, that by amplitude and phaseweighting the RF signals that are supplied to the two ports, the antennabeam may be narrowed in the azimuth plane. FIGS. 6G and 6H illustratethe azimuth and elevation patterns for antenna 200 when an RF signal isapplied (without amplitude or phase weighting) to the three ports of theantenna that connect to the linear arrays 220 that have azimuthboresight pointing directions of 0°, 90° and 180°. Finally, FIGS. 6I and6J illustrate the azimuth and elevation patterns for antenna 200 when anRF signal is applied (without amplitude or phase weighting) to all fourlinear arrays 220 of the antenna. 200 As can readily be seen, theantenna 200 is capable of providing omnidirectional coverage in theazimuth plane.

As noted above, eight-port beam-forming antennas are known in the art,but these antennas are normally implemented as panel antennas that havefour columns (linear arrays) of dual-polarized radiating elements, whereall four columns point in the same direction (i.e., have the sameazimuth pointing angle). Such antennas, however, are not well-suited formany small cell applications in dense urban environments where antennashaving omnidirectional coverage are often preferred so that a singlebase station antenna can provide coverage to the small cell.Omnidirectional beam-forming antennas are also known in the art, butthese antennas used omnidirectional radiating elements that had low gainand limited beam-forming capabilities. As shown above, the base stationantenna 200 is capable of providing omnidirectional coverage while usingdirectional radiating elements (i.e., radiating elements that provideless than 360° coverage in the azimuth plane) may allow for narrowerantenna beams that have much higher gain. As will be explained ingreater detail below, the antenna 200 may be used as a beam-formingantenna and/or as a MIMO antenna. Moreover, when used in LTE-TM8beam-forming mode, the base station antenna 200 may operate inconjunction with an off-the-shelf LTE-TDD eight-port beam-forming radiothat will use digital beam-forming techniques to optimize the amplitudeand phase weights that are applied to the signals received at each port244 of the antenna 200. The off-the-shelf radio may also calculate thecomplex conjugates of the optimized weights for the uplink to generatecorresponding amplitude and phase weights for each port 244 on thedownlink.

The base station antenna 200 may be relatively small, having a diameteron the order of 8 inches and a height of about two feet for an antennaoperating in the 2 GHz frequency range. Such an antenna may be readilymounted on most utility poles and streetlights, and given its smalldiameter, the antenna 200 may blend together with the pole so that it isnot a visual blight. Moreover, in urban environments, there aretypically a small number of entities that own the utility poles such asan electric power company, a government entity (e.g., for streetlights),and a landline telephone company. As such, deploying small cell basestation antenna that are utility pole mountable—such as the base stationantenna 200—may be advantageous since a cellular operator can reach aleasing agreement with one or two entities to obtain locations formounting small cell base station antennas throughout the urban area. Incontrast, panel beam-forming antennas are typically only “stealthy” ifmounted on walls or buildings. Typically, a cellular operator will needto lease mounting locations on walls/buildings one or a few at a time,which makes the transactional costs of the leasing negotiations muchhigher.

With beam-forming antennas, it is typically desirable to reduce thehorizontal spacing between adjacent linear arrays in order to reducesidelobes in the antenna pattern and increase the directivity of theantenna beam. Typically, a horizontal spacing of about 0.5 wavelengthsis desirable for beam-forming antennas. In contrast, when MIMOtransmission techniques are used, it is necessary that the linear arraystransmitting different data streams be relatively decorrelated. Twotechniques that are routinely used to achieve such decorrelation arepolarization diversity and spatial diversity. Cross-polarized signalsmay achieve high levels of decorrelation, although these levels may bereduced somewhat in high-scattering environments. Spatial diversity isachieved by physically spacing the linear arrays apart. Typically, ahorizontal (azimuth plane) spacing of a wavelength or more is desiredfor MIMO antennas. However, the necessary spacing is reduced the morecluttered (high scattering) the environment. Small cell antennas aretypically used in urban environments that have high degrees ofscattering, and hence sufficient decorrelation may be achieved even whenthe linear arrays are relatively close together.

Here, the linear arrays 220 of base station antenna 200 may be locatedin close proximity for purposes of beam-forming. Since the azimuthboresight pointing direction of each linear array 220 is different, thelinear arrays 220 may be sufficiently decorrelated, particularly whenthe antenna 200 is deployed in a dense urban environment. As such, thesmall cell base station antenna 200 may be used as both a beam-formingantenna and a MIMO antenna in some applications. In particular, MIMOtechniques may be used to break up data that is to be transmitted to auser into a plurality of independent data streams, and each of thesedata streams is transmitted to the user via either two or four of thelinear arrays. For example, the antenna 200 may transmit signals using2×MIMO techniques by transmitting a first data stream through the fourlinear arrays 220 at a first polarization while simultaneouslytransmitting a second data stream through the four linear arrays 220 ata second polarization (with both transmissions at the same frequency).

The base station antenna 200 may be well-suited for operation in LTE-TDDand LTE-FDD systems. In some embodiments, the base station antenna 200may be used in LTE-TDD TM8 mode or LTE-FDD TM8 mode. In particular, thebase station antenna 200 may be used in conjunction with a standardoff-the-shelf eight port beam-forming radio 42 to operate in LTE-TDD TM8mode as follows.

During a particular time slot, an RF signal that is transmitted by theuser assigned to the time slot is received at the antenna 200. This RFsignal may be received at the −45° dipoles and the +45° dipoles of allfour linear arrays 220-1 through 220-4. The magnitude and phase of thesub-components of the RF signal that are received at each linear array220 (and at the radiating elements 222 of each linear array 220) willdiffer due to differences in transmission path lengths, fading, theazimuth pointing direction of each array and various other factors.Multiple versions of the transmitted RF signal may be received at one ormore of the linear arrays 220 due to signal reflections off buildings,terrain features or the like that result in multipath transmission. Thesignals received at each of the eight linear arrays 220 are fed to thebeam-forming radio 42. The beam-forming radio 42 uses an optimizationalgorithm to determine amplitude and phase weights to apply to thesignals received at each port 244-1 through 244-8 that optimize someperformance parameter such as signal-to-noise ratio, bit error rate orthe like. The optimization algorithm may be, for example, aninterference rejection combining or maximum ratio combining optimizationalgorithm. The beam-forming radio 42 applies the amplitude and phaseweights determined by the optimization algorithm in demodulating thereceived RF signal. This technique is known in the art as digitalbeam-forming.

The beam-forming radio 42 may determine the complex conjugates of theamplitude and phase weights that maximize the performance parameter forthe received (uplink) signal and may use these complex conjugates as theamplitude and phase weights for transmitting RF signals through thelinear arrays 220 on the downlink.

The beam-forming radio 42 may determine amplitude and phase weights toapply to the signals received at each port 244-1 through 244-8 for eachtime slot in a frame structure of a TDD transmission system. The radio42 may communicate with a different user (or set of users) during eachtime slot in the frame. The radio 42 will periodically perform channelsampling operations to determine the state of the channel(s) between thebase station antenna and the user served during a particular time slot.Based on the results of the channel sampling, the radio 42 may forexample, transmit data to a particular user using beam-forming alone,using beam-forming in conjunction with 2×MIMO transmission techniques orusing beam-forming in conjunction with 4×MIMO transmission techniques.The radio 42 will periodically perform additional channel sampling todetermine if the channel characteristics during each time slot havechanged and revise the transmission techniques used based on the updatedchannel characteristics.

It will also be appreciated that the base station antennas according toembodiments of the present invention may further include a calibrationport and one or more calibration elements. In some embodiments, thecalibration element may comprise, for example, a distributed calibrationelement that may receive signals transmitted by each of the lineararrays of radiating elements. An example of a suitable distributedcalibration element is a leaky coaxial cable. The radio may transmitcalibration signals that are transmitted through one or more of thelinear arrays of the antenna. A different calibration signal may betransmitted through each linear array. The one or more calibrationelements may receive the calibration signals and pass the receivedcalibration signals to the radio through the calibration port of thebase station antenna. The radio may determine the amplitude and phasesof the received calibration signals so that the amplitude and phaseweights may be calibrated to more exactly control the amplitude andphase shifts between the linear arrays.

FIG. 9 is a block diagram of a base station antenna 300 according toembodiments of the present invention that has a calibration port 302 anda distributed calibration element 304. As shown in FIG. 9, thedistributed calibration element 304 may be positioned to receive RFsignals that are transmitted through each of the linear arrays 320 ofdual-polarized radiating elements 322. The signals received through thedistributed calibration element 304 are passed to the calibration port302 and then passed to a calibration port 48 on a radio 42. Eachcalibration signal (i.e., the calibration signals transmitted throughthe different linear arrays 320) may be transmitted at a differentfrequency or may include a unique code so that the receiver at the radio42 can differentiate between the calibration signals to determine thephase shifts therebetween. The radio 42 may use this information toensure that the amplitude and phase weights that are applied to the RFsignals transmitted to the various linear arrays 320 provide optimizedantenna beams.

FIG. 10 is a flow chart diagram illustrating a method of operating abase station antenna according to embodiments of the present invention.The base station antenna includes a plurality of linear arrays ofdual-polarized radiating elements, where at least two of the lineararrays have different azimuth boresight pointing directions. As shown inFIG. 10, operations may begin with the reception of an RF signal from auser through the plurality of linear arrays during a first time slot ofa time division multiplex system (Block 500). This received RF signal ispassed to a beam-forming radio (Block 510). A first set of amplitude andphase weights are then determined, the first set of amplitude and phaseweights including a respective amplitude weight and a respective phaseweight for each linear array at each of a first polarization and asecond polarization (Block 520). The first set of amplitude and phaseweights may then be applied to the received RF signal (Block 530). Asecond set of amplitude and phase weights may then be determined, thesecond set of amplitude and phase weights including a respectiveamplitude weight and a respective phase weight for each linear array ateach of the first polarization and the second polarization (Block 540).The amplitude and phase weights in the second set of amplitude and phaseweights may be the complex conjugates of the amplitude and phase weightsin the first set of amplitude and phase weights. The second set ofamplitude and phase weights may then be applied to one or more RFsignals that are transmitted to the user through the plurality of lineararrays (Block 550).

FIG. 11 is a schematic perspective view of a base station antenna 400according to further embodiments of the present invention. The antenna400 is similar to the antenna 200 that is described above with referenceto FIGS. 6A-8, except that linear arrays 420-1 and 420-2 of radiatingelements 422 are mounted on a first backplane 412-1, while linear arrays420-3 and 420-4 of radiating elements 422 included in the antenna 400are mounted on a third backplane 412-3, and no linear arrays are mountedon backplane 412-2 or backplane 412-4. The third backplane 412-3 isopposite the first backplane 412-1. In other words, the two antennas 200and 400 may be identical except for the locations of the linear arrays420 on the backplanes 412. The antenna 400 may be a bi-directionalantenna that is well-suited for use in tunnels, for outside coveragealong city streets and the like where coverage is only need generallyalong a longitudinal axis as opposed to the omnidirectional coverageprovided by antenna 200. The antenna 400 may comprise an eight-portantenna that connects to an eight-port beam-forming radio 42. The portsare not illustrated in FIG. 11 to simplify the drawing, but may beidentical to the ports 244 of base station antenna 200. The antenna 400may further include a calibration port. The antenna 400 may be capableof generating narrower, higher gain beams than the antenna 200, sincetwo linear arrays 420 are mounted side-by-side on a backplane (i.e., onbackplane 412-1, and also on backplane 412-3).

It will appreciated that many modifications may be made to the antennasdescribed above without departing from the scope of the presentinvention. For example, the base station antenna 200 includes fourlinear arrays 220 that are mounted on the four sides of a supportstructure that has a square horizontal cross-section. In otherembodiments, a base station antenna may be provided that is identical tothe base station antenna 200 except that it includes five linear arraysthat are mounted on a support structure having a pentagon-shapedhorizontal cross-section. In still other embodiments, a base stationantenna may be provided that is identical to the base station antenna200 except that it includes six linear arrays that are mounted on asupport structure having a hexagonal horizontal cross-section.

The present invention has been described above with reference to theaccompanying drawings. The invention is not limited to the illustratedembodiments; rather, these embodiments are intended to fully andcompletely disclose the invention to those skilled in this art. In thedrawings, like numbers refer to like elements throughout. Thicknessesand dimensions of some elements may not be to scale.

Spatially relative terms, such as “under”, “below”, “lower”, “over”,“upper”, “top”, “bottom” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. For example, if the device inthe figures is turned over, elements described as “under” or “beneath”other elements or features would then be oriented “over” the otherelements or features. Thus, the exemplary term “under” can encompassboth an orientation of over and under. The device may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein interpreted accordingly.

Well-known functions or constructions may not be described in detail forbrevity and/or clarity. As used herein the expression “and/or” includesany and all combinations of one or more of the associated listed items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another. For example, a first element could be termed asecond element, and, similarly, a second element could be termed a firstelement, without departing from the scope of the present invention.

That which is claimed is:
 1. A small cell base station, comprising: abase station antenna having first through eighth ports and first throughfourth linear arrays of dual-polarized radiating elements, where atleast two of the first through fourth linear arrays have differentazimuth boresight pointing directions when the base station antenna ismounted for use, and wherein each dual-polarized radiating elementincludes first and second radiators that have respective directionalradiation patterns; a radio having first through eighth radio ports thatare connected to the respective first through eighth ports of the basestation antenna; and a feed network that connects all of the firstradiators of each of the first through fourth linear arrays to the firstthrough fourth radio ports, respectively, and connects all of the secondradiators of each of the first through fourth linear arrays to the fifththrough eighth radio ports, respectively, wherein the radio isconfigured to determine and apply a first set of amplitude and phaseweights to radio frequency (“RF”) signals that are received from therespective first through eighth ports of the base station antenna, andto apply a second set of amplitude and phase weights to RF signals thatare output by the radio to the first through eighth ports of the basestation antenna.
 2. The base station of claim 1, wherein the amplitudeand phase weights of the first set of amplitude and phase weights andthe amplitude and phase weights of the second set of amplitude and phaseweights are each determined on a time slot-by-time slot basis.
 3. Thebase station of claim 2, wherein the amplitude and phase weights of thesecond set of amplitude and phase weights are the complex conjugates ofthe respective amplitude and phase weights of the first set of amplitudeand phase weights.
 4. The base station of claim 1, wherein the azimuthboresight pointing direction of the first linear array is offset fromthe azimuth boresight pointing direction of the second through fourthlinear arrays by about 90 degrees, about 180 degrees and about 270degrees, respectively.
 5. The base station of claim 1, wherein the basestation antenna is configured to transmit as a multi-input-multi-outputantenna using the first radiators of the dual polarized radiatingelements to transmit a first data stream and using the second radiatorsof the dual polarized radiating elements to simultaneously transmit asecond data stream that is different than the first data stream.
 6. Thebase station of claim 5, wherein the first and second data streams arepart of a composite data stream that is transmitted to a user.
 7. Thebase station of claim 1, wherein the base station antenna furtherincludes first through fourth backplanes that together define a tubularreflector assembly.
 8. The base station of claim 7, wherein the tubularreflector assembly has a generally rectangular cross-section in theazimuth plane.
 9. The base station of claim 1, the base station antennafurther comprising a ninth port and a calibration element that isconnected to the ninth port and the radio further comprises a ninthradio port, wherein the ninth port of the base station antenna isconnected to the ninth radio port.
 10. The base station of claim 1,wherein the azimuth boresight pointing direction of the first lineararray is offset from the azimuth boresight pointing direction of thesecond through fourth linear arrays by about 90 degrees, about 180degrees and about 270 degrees, respectively.
 11. The base station ofclaim 1, wherein the base station is configured to operate in a LongTerm Evolution Transmission Mode 8 beam-forming mode.
 12. A small cellbase station, comprising: a base station antenna having first througheighth ports and first through fourth linear arrays of dual-polarizedradiating elements, each of the first through fourth linear arraysconnected to a respective pair of the first through eighth ports, whereat least two of the first through fourth linear arrays have differentazimuth boresight pointing directions when the base station antenna ismounted for use, and wherein each dual-polarized radiating elementincludes first and second radiators that have respective directionalradiation patterns; and a radio having first through eighth radio portsthat are connected to the respective first through eighth ports of thebase station antenna, wherein the radio is configured to determine andapply a first set of amplitude and phase weights to radio frequency(“RF”) signals that are received from the respective first througheighth ports of the base station antenna, and to apply a second set ofamplitude and phase weights to RF signals that are output by the radioto the first through eighth ports of the base station antenna; whereinthe first linear array points in a first direction and the third lineararray points in a third direction that is substantially opposite thefirst direction.
 13. The base station of claim 12, wherein the secondlinear array points in a second direction and the fourth linear arraypoints in a fourth direction that is substantially opposite the seconddirection.
 14. The base station of claim 13, wherein the first directionis angularly offset from the second direction by about 90 degrees. 15.The base station of claim 13, wherein the first direction is the same asthe second direction.
 16. The base station of claim 12, wherein the basestation is configured to operate in a Long Term Evolution TransmissionMode 8 beam-forming mode.
 17. A small cell base station, comprising: abase station antenna having first through fourth linear arrays ofradiating elements that are coupled to respective first through fourthantenna ports; and a radio having first through fourth radio ports thatare coupled to the respective first through fourth antenna ports,wherein the radio is configured to determine and apply a first set ofamplitude and phase weights to radio frequency (“RF”) signals that arereceived from the respective first through fourth antenna ports, and toapply a second set of amplitude and phase weights to RF signals that areoutput by the radio to the first through fourth antenna ports, whereinthe radio is further configurable to transmit an RF signal through atleast a first and second of the linear arrays of radiating elements thathave different azimuth boresight pointing directions to generate anantenna beam.
 18. The small cell base station of claim 17, wherein thefirst and second linear arrays point in opposite directions in theazimuth plane.